An ordinary laser interferometer does not have a detector of an origin position and it can only perform a relative displacement measurement. On the other hand, Japanese Patent Laid-Open Nos. 2007-33317 and 2007-33318 disclose a laser interferometer which has an origin detector. In the method proposed in these references, in addition to a first light source for measuring a position, a second light source whose wavelength or coherency is different from that of the first light source is used. The laser interferometer is provided with the origin detector which defines either one of a point where the phases of each of the interference signals obtained from these two light sources coincide with each other or a point where an intensity of the interference signal obtained from the light source which has low coherency is maximized as an origin.
However, while the position measurement using the first light source is performed with extremely high accuracy, it is difficult to measure the origin position with high accuracy even if it is measured based on the point where the phases coincide with each other or the point where the interference signal is maximized. Therefore, there was a problem that the reproducibility of the origin position was considerably inferior compared to the resolution of the first light source.
This problem can be solved by improving the measurement accuracy of the origin position by defining a point where the phase of the interference signal obtained from the first light source is a certain value, for example zero, as an option of the origin and by defining the option of the origin which is the closest to the measured origin described above as the origin.
If the interferometer is an ideal one, a path of light reflected by a reference mirror and a path of light reflected by a target mirror are symmetrically constituted and there is a position where optical paths are identical for all wavelengths. However, in the interferometer which is industrially used, the thickness of a glass material included in both optical paths is different because of an error of the thickness of a prism and the like. There is also a problem that the variation of the wavelength causes the change of the relationship between the measured origin position and the phase of the interference signal obtained from the first light source.
When the measured origin position is positioned at an intermediate position of two options of the origin which are determined from the first light source, it can not be determined which option may be selected as the origin. There was also a problem that either one of the two options of the origin adjacent to each other was selected as the origin at random, and as a result, the origin position was significantly varies.